Respiratory distress syndrome, the most common pulmonary disorder in newborns, is the leading cause of neonatal death. Successful treatment of this disease depends critically on the continuing provision of adequate ventilatory support. However, infant ventilators in use today are not optimized for the size and needs of the infant, causing high incidences of complications and mortality. In this project, we will evaluate a new type of infant ventilator based upon the technique of "negative-impedance ventilation" (NIV) we recently proposed. Inspiratory airflow and airway pressure are continuously controlled by a fast-responding servo valve which establishes a constant pressure/flow or pressure/volume ratio, simulating respectively a negative resistance or negative elastance (or a combination of both). with lowered airway pressure swings and synchronous airflow pattern, this novel technique promises to eliminate many pitfalls of mechanical ventilation. In this project we will examine the immediate physiological effects of NIV compared to CMV in three animal models of neonatal pulmonary disease: 1) lung lavage model of hyaline membrane disease; 2) surfactant therapy model; 3) oleic acid-induced lung injury. The results of this investigation will lay the groundwork for further clinical evaluations and physiological studies of synchronized assisted ventilation in the future.